1. Field of the Invention
The present invention relates to a misalignment detection device which determines whether an image is identical to a reference image, and an optical writing apparatus, an image forming apparatus and a photocopier which use the misalignment detection device.
2. Discussion of the Background
Recently, image forming apparatuses having multiple optical writing systems, which can irradiate light beams to form a latent electrostatic image on a photoconductor serving as an image bearing body, have been developed.
Published unexamined Japanese Patent Application (hereinafter referred to as JOP) No. 2000-267027 discloses an image forming apparatus capable of forming images on a paper having a large width such as A1 and A0 sizes. The image forming apparatus includes a low-cost and high definition optical writing system having a large width by separately scanning multiple optical writing systems so that the scanned light beams are lined in the main scan direction (this optical writing system is referred to as a separate scanning optical system).
JOP No. 6-18796 discloses an image forming apparatus having multiple optical writing systems and multiple photoconductors serving as image bearing members to obtain a multi-color image. This is generally referred to as “tandem image forming apparatus”. In the apparatus, a multi-color image is obtained as follows. The multiple optical writing systems irradiate the respective image bearing members with a laser beam for forming latent electrostatic images thereon. The latent images are developed with respective color toners. The toner images are overlaid on a transfer material such as a transfer belt or paper.
Further, JOP No. 6-1002 discloses another image forming apparatus having multiple optical writing systems and a sole photoconductor. In this apparatus, a multi-color image is obtained by irradiating the sole photoconductor with a light beam emitted by each of the multiple optical writing systems to form latent electrostatic images; and developing the images with respective color toners.
However, in the image forming apparatus disclosed in JOP No. 2000-267027, the multiple laser beams used take separate light paths when scanning. Therefore, the relative positions of the multiple laser beams to each other vary depending on temperature distributions in the writing systems, environmental variances and variances of wavelengths of the laser beams caused by the temperature of the laser diodes. This causes problems of white streaks and black streaks at joint points, resulting in serious deterioration in image quality. The laser beam positions also vary in the cases of the image forming apparatuses disclosed in JOP Nos. 6-18796 and 6-1002 and this leads to a problem of misalignment of the overlaid color images, resulting in deterioration of image qualities such as non-uniform coloring and blur.
To address these problems, Japanese Patent No. 3253227 discloses a beam position correction method. In this method, a charge coupled device (CCD) functioning as an image sensor is provided to determine whether the registration marks formed on the transfer belt for correcting the positions of the beams are correctly aligned. Further, to improve the misalignment detection precision level, multiple light sources are symmetrically located relative to the registration marks. Thereby the registration marks can be irradiated with substantially uniform amount of light from any angle, resulting in minimization of the error caused by seesaw movement of the transfer belt.
However, the technology disclosed in this patent requires multiple light sources of light emitting diodes (LED) and thereby causes a cost problem. In addition, a condenser lens for collimating flux of light from LED chips is also proposed in the patent but is also costly. Furthermore, a focal length of the detection optical system is minimally about 8 mm. Therefore, when an object and the image thereof have the same size, the minimum conjugation length is about 35 mm by calculation (8 mm times 4+the distance between the principal points of the lenses). This is a large barrier to miniaturization of the device since the total height thereof is almost 40 mm while taking the size of the CCD and the thickness of the substrate into consideration.
A mark for use in detecting misalignment can be formed on a photoconductor. In this case, since photoconductors typically have a mirror surface where diffuse reflection rarely occurs, the regular reflection light from the mark needs to be securely focused on an image sensor. When the regular reflection light from the mark for detecting misalignment does not reach the image sensor, the image sensor misjudges that the mark is total black. The reason therefore is that the mark is formed by toner and the mark formed with black toner has little diffuse reflection.
Because of these reasons, the need still exists for a compact-sized and low-cost misalignment detection device which can determine whether an image is misaligned from its correct position with high precision.